A method for the visualization of image data via electrostatic image such as electrophotography is utilized in various fields at present. In electrophotography, an electrostatic latent image is formed on a photoreceptor via charging step and exposure step. The electrostatic latent image thus formed is then developed with a developer comprising a toner. The image thus developed is then visualized via transfer step and fixing step. The developer for use in electrophotography is divided into two types, i.e., two-component developer comprising a toner and a carrier and single-component developer comprising a toner singly, such as magnetic toner. In the two-component developer, the carrier serves to agitate, carry and charge the developer and thus two components for the developer each has a different function. Thus, the two-component developer features a good controllability and is widely used at present.
In particular, a developer comprising a carrier which has a core coated with a resin has an excellent controllability and can be relatively easily subjected to improvement in environmental dependence and age stability. As the developing method there was once used cascade method. At present, a magnetic brush method using a magnetic roll as a developer carrier is mostly used.
The magnetic brush method using a two-component developer is disadvantageous in that the chargeability deterioration of the developer causes a drop of image density and a remarkable stain on the background and the attachment of the carrier to the image gives defective image or causes excessive consumption of the carrier or gives uneven image density. The chargeability deterioration of the developer can easily occur when the toner components are fixed to the coated layer of the carrier or the coated layer is peeled. Further, if the coated layer is nonuniform, it tends to cause stain on the background when the environmental factors such as humidity and temperature are changed, when the toner is additionally supplied or when the toner concentration becomes high.
The mechanism of the attachment of the carrier to the image is considered as follows. In some detail, when the coated layer is nonuniform or peeled, causing a drop of the electrical resistance of the carrier, induced electric charge is injected into the image zone to cause the carrier to be attached to the image, particularly to the edge of the image.
The adjustment of the electrical resistance of the carrier is proposed in JP-A-2-37366 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-5-150565. However, these publications have no reference to countermeasure against the drop of the electrical resistance of the carrier due to the peeling of the coating resin.
As a coating method for fixing the coated layer more uniformly and firmly there has been proposed a method which comprises mixing a magnetic core with a powdered coating material in a dry condition and, heat-melting the mixture to form a coated layer. However, taking into account the recent trend toward smaller particle diameter of toner grains and lower melting point of toner material for higher image quality, the foregoing proposed method cannot necessarily exert a sufficient effect of solving the foregoing problems.